there's a site that explains more about DXM quantities and side effects:
Abuse of DXM can lead to intoxication and overdose. Learn the symptoms of an overdose and how to handle it, and how to get help so you can avoid a serious threat to your health.
americanaddictioncenters.org
Also, we have a
thread that has a similar question.
There's a quote that says: "At 1500 mg or more, a person is likely to overdose on dextromethorphan". Mixing it with alcohol would likely succeed your CTB. Although the amount you have available now is more than enough to that. But do your research about the consequences of a failed attempt and it's effects, because I'm not really sure of it.
May you find the peace you're searching for. Good luck <3